1. Field of the Invention
This invention relates to injection moulding apparatus.
2. Prior Art
British Patent Specification No. 2139548 describes a process of producing an injection moulding comprising introducing plastics material into a mould space and at one or more selected positions separately introducing a pressurized gas into the stream of plastics material filling the mould to create a gas containing cavity in the plastics material, maintaining the pressurization in the gas and utilising the pressurized gas in the plastics material to cause the plastics material to flow throughout the mould space with the gas containing cavity within the plastics material, the cavity thereby extending with the plastics material, and when the plastics material has extended over the whole of the mould space, continuing to maintain the gas pressure within the gas containing cavity to hold the plastics material positively against the mould surface as the plastics material solidifies and cools until the moulding can itself sustain the form dictated by the mould surface.
In one embodiment described the pressurized gas is introduced at a position within the mould space, and in an alternative embodiment the gas is introduced upstream of the mould space adjacent the outlet end of a screw ram for introducing the plastics material. In each case the gas is introduced through a passageway subsequently to the outlet end of the passageway being immersed in the plastics material, the gas entering the plastics material and forming the gas containing cavity therein.
At the end of the moulding cycle, the gas pressure within the gas containing cavity is relieved before the mould is opened. In each embodiment described in British Patent Specification No. 2139548, a valve is opened to allow the gas to flow back through the passageway through which it was introduced to create the cavity, the gas emitted conveniently passing to atmosphere. This arrangement works well in the embodiment of Specification No. 2139548 in which the outlet end of the passageway is positioned upstream of the mould space and the passageway is relatively short.
However, in the case of the embodiment in Specification No. 2139548 in which the outlet end of the passageway is positioned within the mould space, it has been found to be more difficult to evacuate the cavity sufficiently. There is also a tendency for the outlet end of the passageway to become blocked by plastics material which must first immerse the outlet end of the passageway before gas is introduced and whilst the plastics material is still molten.
For the production of some mouldings, especially mouldings of complex design, it is desirable to introduce the pressurized gas at one or more selected positions either within the mould space alone or both within the mould space and upstream of the mould space, and subsequently to evacuate the gas from the moulding at each position. Such a situation may arise, for example, when it is desired to employ a hot runner system or the choice of gate position is restricted by the desire to avoid excess use of gas ribs. The present invention makes it possible to introduce the gas at any selected position without the aforesaid disadvantages.